The present invention relates to a nozzle for discharging molten metal used in a casting device.
Alumina-graphite and zirconia-graphite have been popularly used as the material of the nozzles for discharging molten metal such as a submerged nozzle for continuous discharging. These materials show high corrosion resistance against molten steel, but they have the defect that they tend to invite deposition of base metal because of their high heat conductivity. Especially in the case of steel with high aluminum content such as aluminum killed steel, there tends to take place blockage of a nozzle bore of the nozzle due to deposition of aluminum oxides such as Al.sub.2 O.sub.3, necessitating interruption of casting operation.
Countermeasures such as improvement of preheating conditions and heat insulation have been taken, with an appreciable effect, against the blockage due to the deposition of the base metal.
On the other hand, for preventing the blockage due to aluminum oxides, there is employed a slit type submerged nozzle in which a porous refractory is provided on an inner surface portion defining the nozzle bore to introduce an inert gas through the porous refractory. This slit type submerged nozzle, however, has the following problems.
Since it is difficult to provide slits close to a discharging port of molten steel, it is hardly possible to prevent the deposition of metal and the blockage at the area near the discharging port. Also, carbon content of the porous refractory is gradually oxidized away while SiO.sub.2 is reacted with the C--CO reducing atmosphere to become SiO and dissipated in that form as the discharging operation is conducted repeatedly. This results in an increased gas permeability of the porous refractory, making it difficult to control the permeation rate of inert gas. Further, increased feed of inert gas encourages formation of pinholes in the cast steel.
Many attempts have been made for preventing the blockage by improving the nozzle material. For instance, Japanese Patent Application Kokai (Laid-Open) No. 57-71860 proposes a method in which a CaO-graphite type refractory is used and the nozzle component is reacted with Al.sub.2 O.sub.3 in molten steel to produce a CaO-Al.sub.2 O.sub.3 type low-melting material to thereby use away Al.sub.2 O.sub.3 which is inclined to deposit on the inner surface.
However, the CaO-Al.sub.2 O.sub.3 type materials don't always turn out a low-melting material; there could rather be formed a high-melting material, which becomes the core of the deposition of Al.sub.2 O.sub.3 to expedite the blockage.